1. Field of the Art
This invention relates to a rear focus type zoom lens particularly suitable for use on a video camera with an autofocusing mechanism.
2. Description of the Prior Art
The main stream of conventional zoom lens construction has been to employ a second lens group for zooming operation, in combination with a third lens group or the so-called compensator which makes correction for shifts of focal point resulting from a zooming operation, and a first lens group which is movable along the optical axis for focusing purposes. However, the lens construction of this sort has a difficulty that an autofocusing mechanism is burdened with a great load when moving the first lens group for focusing purposes since the first lens group is usually large in diameter and heavy in weight. In this connection, as described in U.S. Pat. No. 4,256,381, for example, there has been proposed and known a lens construction employing, for focusing purposes, a fourth lens group which is small and light as compared with the first lens group. In this known lens construction, the first and third lens groups are held stationary, the second lens group is movable for zooming operation, and the focusing fourth lens group is also movable along the optical axis to maintain a plane of image despite the shift of focal point resulting from the zooming movement of the second lens group.
The lens construction which incorporates a fourth lens group in the above-described manner is advantageous in that the load on the autofocusing mechanism can be lessened. However, since the stop is fixed in the vicinity of the third lens group during a zooming operation, it is difficult to reduce the size of the bulky first lens group with the greatest outer diameter, barring efforts to reduce the size of the zoom lens as a whole for the sake of compactness.